Answer: Pope Pius V continued to wear his Dominican habit under his papal robes, thus establishing the tradition that popes wear white. The custom continued after Pope Pius V, and continues down to this present day. The only Vatican stamp we have for Pope Pius V is from an image from the altarpiece of the Chapel of the Rosary in Santa Croce di Bosco Manengo painted around 1597. This painting commemorates the victory in the Battle of Lepanto.



5th Centenary of Birth Of St. Pius V
Scott 1258 & 1259 (2004)

See also
Why Does the Pope Wear White? Catholic Apologetice (website) https://aquinasonline.com/why-does-the-pope-wear-white/